1. Field of the Invention
The present invention relates to a liquid crystal display (LCD), and more particularly, to a driving unit and a timing controller which control an LCD to drive gate lines included in the LCD in units of a predetermined number of gate lines, and a driving method used by the LCD.
2. Description of the Related Art
A conventional liquid crystal display (LCD) applies an adjustable voltage to a material having an anisotropic permittivity injected between two substrates to adjust the amount of light transmitted through the substrates, thereby obtaining a desired image. The LCD includes a plurality of scan lines transmitting gate select signals and a plurality of data lines crossing the scan lines and transmitting color data, i.e., image data. The LCD also includes a plurality of pixels arranged in a matrix pattern, disposed at intersections of the scan lines and the data lines, and connected to one another by the scan lines, the data lines, and switching devices.
To transmit image data to each of the pixels of the LCD, on/off signals are sequentially transmitted to gate lines (scan lines). Then, the switching devices connected to the gate lines are sequentially turned on/off. Simultaneously, an image signal to be transmitted to a row of pixels corresponding to a gate line is converted into a gradation voltage that can take on a plurality of voltage levels, and the gradation voltage is applied to each data line. Here, during one frame cycle, gate signals are sequentially transmitted to all the scan lines such that pixel signals are transmitted to all rows of pixels. As a result, an image of one frame is displayed.
When an electric field is continuously applied to the LCD in one direction, characteristics of the LCD deteriorate due to inherent characteristics of a liquid crystal material. Therefore, the polarity of a common voltage must be inverted. In other words, if a positive voltage is applied to a pixel in a frame, a negative voltage should be applied to the same pixel in another frame. Consequently, the positive and negative voltages are repeatedly applied to the same pixel in an alternating fashion.
A method of inversion-driving an LCD includes a frame inversion driving method in which the polarity of a common voltage is inverted in units of frames, a line inversion driving method in which the polarity of the common voltage is inverted in units of gate lines whenever each gate line is scanned, and a dot inversion driving method in which the polarity of the common voltage is inverted in units of pixels.
Intermediate gradation screens, such as a screen displayed when Windows is closed, of LCDs using the dot inversion driving method experience shake. In addition, since data lines are driven at large amplitude in the dot inversion driving method, high power consumption is required. Thus, LCDs using the dot inversion driving method are seldom used for portable terminals.
FIG. 1A illustrates gate lines driven using the frame inversion driving method. Referring to FIG. 1A, the polarity of a common voltage Vcom is inverted in units of frames. A positive common voltage is applied to an Nth frame to sequentially scan all the gate lines for the Nth frame, and image data of the Nth frame is output. Then, a negative common voltage is applied to an N+1th frame to sequentially scan all the gate lines for the N+1th frame. If 60 frames are scanned per second, an LCD inverts the polarity of the common voltage Vcom every 1/60 of a second.
The LCD consumes power whenever the polarity of the common voltage Vcom is inverted. Thus, a frame inversion driving method in which the polarity of the common voltage Vcom is inverted less frequently has lower power consumption. However, since the polarity of all the gate lines is inverted each frame, all the gate lines have the same polarity. Therefore, a difference in liquid crystal transmittance of two frames is easily recognized, causing the screen to flicker. Thus, the frame inversion driving method is rarely used.
FIG. 1B illustrates gate lines driven using the line inversion driving method. Referring to FIG. 1B, the polarity of a common voltage Vcom is inverted whenever each of the gate lines for an Nth frame is scanned. For example, if positive-polarity data is transmitted to odd numbered scan lines, negative-polarity data is transmitted to even numbered scan lines. When an N+1th frame is scanned, the polarity of the even numbered scan lines and that of the odd numbered scan lines are inverted, thereby preventing deterioration of the liquid crystal material. In addition, since the polarity of the common voltage Vcom is inverted in units of lines, the problem of screen flickering can be solved.
However, since the polarity of the common voltage Vcom is inverted for each gate line, high power consumption is required. Such high power consumption puts an LCD using the line inversion driving method at a great disadvantage when the LCD is to be used in portable devices constrained by power. For example, if the LCD has 480 gate lines, the LCD inverts the polarity of the common voltage Vcom once every 1/(60×480) of a second, consuming much power.
FIG. 1C illustrates gate lines driven using an n-line inversion driving method. Referring to FIG. 1C, after n gate lines are scanned, the polarity of a common voltage Vcom is inverted. Then, another n gate lines are scanned. After a frame is scanned in this way, the polarity of the common voltage Vcom applied to the next frame is opposite to that applied to the previous frame.
Since the gate lines are scanned in units of n lines using the common voltage Vcom of the same polarity and then the polarity of the common voltage Vcom is inverted, the n-line inversion driving method can reduce power consumption to 1/n that used in the line inversion driving method. In other words, if the polarity of the common voltage Vcom is inverted every three lines, the polarity of the common voltage is inverted once every 3/(60×480) of a second. However, since the polarity of the common voltage Vcom is inverted every n adjacent lines, the n-line inversion driving method results in flickering.
FIG. 2 is a graph illustrating power consumption of each of the inversion driving methods. Referring to FIG. 2, while 1.35 mA are consumed in the frame inversion driving method, 1.85 mA are consumed in the line inversion driving method. It can be seen that a 2-line inversion driving method consumes 1.60 mA, which is between 1.35 mA of the frame inversion driving method and 1.85 mA of the line inversion driving method. On the other hand, a 3-line inversion driving method consumes 1.47 mA. Therefore, it can be understood that far less power is consumed in a 2- or greater line inversion driving method than in the line inversion driving method. However, when the 2 or more line inversion driving method is used, a number of adjacent lines have the same polarity, and thus the problem of flickering emerges.